Living together is pretty accepted way for modern couples to test a relationship before marriage. So shouldn’t there be a way for modern computer users to test Microsoft’s Windows Vista before making the deep commitment of buying and installing it on their PCs?

There is, using virtualization software. Virtualization is one of the hottest server-side trends today. The technology lets IT managers run multiple applications, with each encapsulated in its own “virtual machine” — a setup that protects them from crashing one another and minimizes security risks. By letting servers safely run multiple workloads, IT managers can save bucketloads on hardware purchases.

Maximizing CPU usage is less important for desktop users. But virtualization can still be useful, letting users try out applications and even operating systems without having to formally install them.

There are three main virtualization options, all of which support Vista to varying degrees. All are free or offer free trial versions. We take a quick look at each, and then explain issues potential Vista testers face now that the operating system is completed but not yet released to the public.

VMware Server and Player: Feature-rich and free

Longtime virtualization market leader VMware Inc. offers both its VMware Server and VMware Player products to users for free. Using both products is necessary, says Srinivas Krishnamurti, director of product management at VMware, since VMware Server actually creates the Vista “guest” — also called a “virtual machine” — in a process very similar to installing the operating system on a PC. Users then install VMware Player on either Linux or any version of Windows up to and including XP, to run the Vista virtual machine.

Going with VMware has several advantages, says Krishnamurti. Users can create Vista virtual machines for either 32- or 64-bit CPUs, and those with multicore PCs can allocate up to two CPUs worth of processing power to a single virtual machine. VMware also lets users create a Vista virtual machine once and run it on different PCs without tweaking. Finally, Krishnamurti claims that VMware, as the veteran in the marketplace, handles device drivers very well. “We’ve tested this pretty extensively,” he said. “But if you plug in some random USB device, and it doesn’t work, we want to know about it.”

Krishnamurti does admit that the latest official versions of its Server and Player products only support USB 1.1, though beta versions out now do offer USB 2.0 support. Those updates will be released officially by the first half of next year, probably after Vista’s Jan. 30 launch to consumers.

Parallels Workstation: Fast and furious

Another offering is Parallels Workstation for Windows & Linux from Parallels Inc. The Renton, Wash.-based firm made a splash earlier this year when Workstation’s sister product for the Mac was the first to allow Intel Mac owners to run Windows simultaneously with OS X. (VMware has subsequently released a similar product, while Apple’s Boot Camp lets users run either Windows or OS X, but not both at the same time.)

Parallels announced last week that the latest update to Workstation, Version 2.2, will run Vista virtual machines. Workstation 2.2, which leverages virtualization technology built into newer AMD and Intel processors for faster performance, will be able to run Vista Ultimate and Vista Business, which cost US$399 and $299, respectively.

Parallels Workstation itself costs $49.99, though it can be downloaded for a free 15-day trial.

But why pay for Parallels when VMware comes for free? For one thing, Benjamin Rudolph, marketing manager for Parallels, claims his company’s software runs much faster. “We think their product is a dump truck and ours is like a pickup truck,” he said. “Ninety-nine out of a hundred times, we offer all that customers need.”

Zippier performance was also the assessment of our sister publication InfoWorld in its August review of Parallels Workstation.

Parallels’ product suffers from some of the same disadvantages that VMware’s does. There is no USB 2.0 support today. There may be driver issues. And unlike VMware, Parallels does not support 64-bit guests or hosts at all now, nor can it delegate more than one CPU to a guest or virtual machine. All of those features will be present in the next major update to Parallels, which is due in the first half of next year, Rudolph said.

Microsoft Virtual PC 2007: A blend of both

Microsoft got into virtualization when it bought Virtual PC in 2003. After VMware made its entry-level products free earlier this year, Microsoft followed suit, making Virtual PC 2004 free for all.

While Virtual PC 2004 doesn’t run on Vista or support Vista virtual machines, Virtual PC 2007, currently in beta, does. Detailed instructions on downloading VPC 2007 and setting up Vista in a virtual machine are at Microsoft’s Vista blog.

Virtual PC 2007 blends the capabilities of the products mentioned above. Like VMware Player, VPC 2007 can run on 32- and 64-bit PCs, though it can only create 32-bit virtual machines at the moment.

Like Parallels, Virtual PC uses Intel’s and AMD’s chip virtualization for faster performance. And while Virtual PC can support a wide range of guest operating systems, it can itself only be installed on PCs running Windows as the base operating system, not Linux.

One similarity between all three products is their inability to support 3-D accelerated graphics. That means you won’t be able to test Vista’s Aero graphical user interface or play the latest first-person shooter video games. In general, virtualized interfaces tend to look rougher and “paint” more slowly than nonvirtualized ones.

Getting your hands on a test version of Vista

Let’s say that you’re now convinced to take Vista for a test spin. Is it too late, now that Microsoft has closed its beta programs? Not at all. The release candidates and betas of Windows Vista, while not available from Microsoft anymore, will still work until June of 2007. Microsoft says that millions of people have downloaded and tested Vista. It’s likely that some of your friends or co-workers have copies. Craigslist and eBay certainly have copies available.

Otherwise, you can wait until Jan. 30, when consumer versions of Vista will be available in stores. According to Computerworld.com’s Scot Finnie, our in-house Windows Vista expert, users can install and run Vista as a virtual machine on top of, say, Windows XP for 30 days without activating Vista. But be sure you know the details of your store’s return policies, such as what the time limit is and whether you’ll have to pay a restocking fee.

Microsoft officially recommends that users interested in testing Vista virtually buy the Windows Vista Enterprise Edition, which grants users the right to install one copy of Vista on a physical machine and up to four times in a virtual machine on the same device for the same user. But Enterprise Edition is available only to corporate volume license customers, putting it out of reach of any hobbyist or small business owner. And postings on Microsoft’s Virtual PC 2007 newsgroup seem to indicate that there’s nothing technically preventing you from buying and testing other versions of Vista using Virtual PC.

A final alternative

If you’re only interested in testing Vista and don’t think you’d ever want to use a virtualization program to test other software, there is an easier